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281.
282.
Coupling the JNLT with the Keck telescope is of considerable interest. Further enhancement may be possible with auxiliary small telescopes, as planned for ESO's VLT. Current plans for installing the optical very large array at Mauna Kea provide opportunities for extra OVLA telescopes near the JNLT.A coudé field slicer is proposed for interferometric observing of a reference star together with the main object. Additions to the JNLT coudé spectrograph are also suggested for use as a speckle camera with multiple spectral channels.Paper presented at the Symposium on the JNLT and Related Engineering Developments, Tokyo, November 29–December 2, 1988. 相似文献
283.
The behavior ofp-modes of high degree and high order in the homogeneous compressilbe model is examined. The second-order differential equation of Pekeris is used to construct asymptotic expansions near the centre and near the surface, which are singular points, and near the turning point of that equation. An equation for the frequencies is obtained by requiring the continuity of the asymptotic solutions and of their first derivatives. Numerical applications are considered. 相似文献
284.
Denis Mercier Samuel Étienne Dominique Sellier Marie‐Françoise André 《地球表面变化过程与地形》2009,34(13):1772-1789
This paper evaluates the paraglacial evolution of a sediment‐mantled slope in a polar maritime environment. The intensity of paraglacial processes is estimated through quantification of erosion and dating of field sectors with the help of photographic archives. Gully erosion has been estimated using morphometric parameters and by surveys of vegetation cover. The rapid melting of dead‐ice cores controls gully formation. This leads to slope form modification: gully profile gradients are reduced from a mean of 35° to a mean ranging between 10° and 15°. Profile evolution results from the collapse of glacier lateral moraine. All data (mean slope angle of individual gullies, frequency distribution of slope angles, fractional distance to the apex, gullying index, volume of debris mobilized, vertical erosion rate) tend to increase with increasing deglaciation age and the duration of paraglacial activity. Vegetation colonization is a response to stabilization of the ground surface and the drying up of the ground surface due to dead‐ice melting. The full sequence of paraglacial slope adjustment (gully incision‐stabilization) may occur rapidly at the study site, i.e. within two decades. Finally, a lateral morphogenic sequence is proposed showing the importance of paraglacial processes at the onset of the deglaciation. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
285.
Denis Gapais Florence Cagnard Frédéric Gueydan Pierre Barbey Michel Ballèvre 《地学学报》2009,21(3):188-194
Orogenic belts involving initially hot lithospheres, as exemplified by accretionary-type orogens, commonly show distributed deformation and retrograde PT paths with a concomitant decrease in pressure and temperature. Paths may track geotherms, indicating thermal equilibrium, consistent with slow strain and exhumation rates, limited strain localization, and consequently limited topographical gradients and distributed erosion. Such patterns are more common in Precambrian times than in younger periods of the Earth history. In contrast, orogens involving initially stiff lithospheres show exhumation PT paths that track isothermal decompression reflecting high strain rates along major shear zones, a feature typical of Phanerozoic collision belts. Field evidence, analogue and numerical models emphasize that strain localization has first-order consequences for the tectonic evolution of orogenic zones including structure, metamorphism, exhumation processes, topography, erosion and sedimentation modes. 相似文献
286.
Experimental evidence is reported for Fe2+ disproportionation in Al-free perovskite (Pv), when submitted to large temperature gradients (i.e., under off-equilibrium
conditions) in a laser heated diamond anvil cell (LHDAC). To enable this effect, the experimental procedure was designed to
produce large radial and axial temperature gradients. In the Pv and ferropericlase (Fp) assemblage synthesized after dissociation
of natural olivine, the three chemical states of iron (i.e., Fe0, Fe2+ and Fe3+) could be evidenced by electron probe microanalysis (EPMA), through variations of oxygen contents attached to the Fe cations.
Despite inherent difficulties for determination of O-contents and Fe3+/ΣFe ratios using EPMA, we recorded significant changes in iron oxidation state across the laser-heated strip. These changes
are correlated with variations in composition for the major elements (Fe, Mg, and Si), which evidences that the Pv/Fp assemblage
experienced large segregation under the strong temperature gradients. Grains of metallic iron were detected in parts of the
laser-heated strip coexisting with a Pv phase with Fe/(Mg + Fe) = 6 at% and most of its iron as Fe3+. This Fe2+-disproportionation reaction involves insertion of Fe3+-defects in the Pv lattice. This Fe3+-bearing Pv phase is presumably unstable and decomposes into a mineral assemblage including magnesioferrite, which is detected
at the border of the laser-heated strip. 相似文献
287.
The general subject of this paper is subglacial deformation beneath Breiðamerkurjökull, a surging Icelandic glacier. More specifically it discusses the evolution and the role of fluid pressure on the behaviour of subglacial sediments during deformation. During Little Ice Age maximum, the two outcrops studied, North Jökulsarlon (N-Jk) and Brennhola-Alda (BA), were located at 2550 m and 550 m respectively from the front of the Breiðamerkurjökull. Sedimentological analysis at the forefield of the glacier shows thick, coarse glaciofluvial deposits interbedded with thin, fine-grained shallow lacustrine/swamp deposits, overlain by a deformed till unit at N-Jk. BA outcrop shows fine-grained shallow lacustrine/swamp deposits overlain by a deformed till unit. The sequence of deformation events from one outcrop to the other is similar. First, major thrust planes, which were rooted in shallow lacustrine/swamp deposits developed by glacially induced simple shear. Next, the thrusts were folded, indicating the deformation of hydroplastic sediment assisted by moderate fluid pressure. Then clastic dyke swarms crosscut the sedimentary succession, proving that fluid overpressure caused hydrofracturing associated with fluidisation. Finally, as water escaped from the glacier bed, fluid pressure dropped, and normal faulting occurred in brittle-state subglacial sediments. Fluid-pressure variations are related to glacier dynamics. They control the deformation sequence by modifying subglacial rheological behaviour and the nature of the subglacial tectonism. 相似文献
288.
Claire M. C. Rambeau Denis Baize Nicolas Saby Virginie Matera Thierry Adatte Karl B. Föllmi 《Environmental Earth Sciences》2010,61(8):1573-1585
Cadmium (Cd) is a highly toxic element and its presence in the environment needs to be closely monitored. Recent systematic
surveys in French soils have revealed the existence of areas in eastern and central France, which show systematically high
cadmium concentrations. It has been suggested that at least part of these anomalous levels are of natural origin. For the
Lower Burgundy area in particular, a direct heritage from the Jurassic limestone bedrock is highly suspected. This potential
relationship has been studied in several localities around Avallon and this study reports new evidence for a direct link between
anomalously elevated cadmium contents of Bajocian and Oxfordian limestone and high cadmium concentrations in deriving soils.
Soils in this area show cadmium concentrations generally above the average national population values, with contents frequently
higher than the ‘upper whisker’ value of 0.8 μg g−1 determined by statistical evaluation. In parallel, limestone rocks studied in the same area exhibit cadmium concentrations
frequently exceeding the mean value of 0.030–0.065 μg g−1 previously given for similar rocks by one order of magnitude, with a maximum of 2.6 μg g−1. Mean ratios between the cadmium concentrations of limestone bedrock and deriving soils (Cdsoil/Cdrock), calculated for different areas, range from 4.6 to 5.7. Calculations based on the analyses of both soils from a restricted
area and fragments of bedrock sampled in the immediate vicinity of high-concentration soils are around 5.5–5.7. Cdsoil/Cdrock is useful in determining the potential of soils in Lower Burgundy to reflect and exacerbate the high concentrations of cadmium
present in parent bedrocks. 相似文献
289.
290.
An Inceptisol A-horizon from Hawaii was subjected to a series of reduction-oxidation cycles—14 d cycle length over a 56 d duration—across the “soil-Fe” [Fe(OH)3.Fe2+(aq), log Ko = 15.74] equilibrium in triplicate redox-stat reactors. Each reducing event simulated the flush of organic C and diminished O2 that accompanies a rainfall-induced leaching of bioavailable reductants from the forest floor into mineral soil. The soil contained considerable amounts of short-range ordered (SRO) minerals (e.g., nano-goethite and allophane) and organic matter (11% org-C). Room temperature and cryogenic 57Fe Mössbauer spectroscopy showed that the iron-bearing minerals were dominated by nano- to micro-scale goethite, and that ferrihydrite was not present. Over the four full cycles, fluctuations in Eh (from 200 to 700 mV) and pFe2+ (from 2.5 to 5.5) were inversely correlated with those of pH (5.5 to 4). Here, we focus on the solubility dynamics of the framework elements (Si, Fe, Ti, and Al) that constitute 35% of the oxygen-free soil dry mass. Intra-cycle oscillations in dissolved (<3 kDa) metals peaked during the reduction half-cycles. Similar intra-cycle oscillations were observed in the HCl and acid ammonium oxalate (AAO) extractable pools. The cumulative response of soil solids during multiple redox oscillations included: (1) a decrease in most HCl and AAO extractable metals and (2) a transformation of SRO Fe (as nano-goethite) to micro-crystalline goethite and micro-crystalline hematite. This may be the first direct demonstration that Fe oxide crystallinity increases during redox oscillations—an a priori unexpected result. 相似文献